Term used to describe the condition of a persons manhood after it has been subjected to a long night of sexual intercourse (usually without a condom), or over-enthusiastic masturbation. Normally red, throbbing and sore to the touch.

Oli: Did you have a good time with that slut last night then?
Mark: Yeah but my dick is so sore today.
Oli: Chlamydia?
Mark: Nah mate probs just a severe case of athletes penis.

The Urban Dictionary T-Shirt

A case where, due to excessive exercise, a man's penis is smaller than normal. Unlike athlete's foot, athlete's penis is not caused by fungus infections. It happens because all of the blood that would usually keep a man's penis his normal size is being used in his muscles. This is not contagious, but very demoralizing to said man.

Person A: "Ah man, it sucks Harrison got pantsed in from of Allie!"
Person B: "Yeah that was the worst case of Athlete's Penis I've ever seen! I don't know if he'll ever be able to live it down..."